We present parameter-free calculations of the frequency-dependent dielectric function in order to understand the optical properties of Ge and Si nanoparticles. The calculations are based upon the independent-particle approximation and a pseudopotential-plane-wave method. The nanoparticles are described by clusters of up to 363 atoms. Their surfaces are passivated by hydrogen atoms. We study the size dependence of the resulting optical spectra and, especially, of the oscillator strengths of transitions near the absorption edge. In the framework of an effective-medium theory the change of the optical properties due to embedding in other environments is discussed in the light of recent measurements, e.g., of Ge nanocrystals in a sapphire matrix.

• Received 31 July 2001

DOI:https://doi.org/10.1103/PhysRevB.65.155328